In the measurement of ballistic projectile velocity, it is common to use a pair of photodetectors spaced a known distance apart to signal to a timer that the projectile has passed each detector. The interval of time measured by the timer and the known distance are then used to compute an instrumental velocity. The photodetectors normally operate by having the receiving elements (typically photodiodes or phototransistors) placed so that they receive steady illumination from incandescent lamps or another steady light source such as the sky or other broad source of light. The receiving element, the light source and the expected projectile path are arranged so that the projectile passes between the light source and the receiving element. The passage of the projectile causes a momentary decrease in the light intensity received by each receiving element. The electrical response of each receiving element is amplified and processed to provide appropriate start and stop signals to the timer.
The photodetector assemblies are typically called "screens" in the ballistics field because they replace physical conducting grids or screens which were used in earlier devices to signal passage of a projectile. Screens utilizing incandescent light sources were typically called "Lumiline" screens because the lamp most commonly used was a Lumiline lamp manufactured by the General Electric Company and others. Screens utilizing ambient light from the sky viewed through an optical slit or other mask were commonly called "sky screens". At the present time, the screens represent the most critical part of a velocity measuring system. The present state of the art in counting and computation circuits required for the timing and time-to-velocity conversion are far ahead of the technology of the screens.
U.S. patent application Ser. No. 840,254, filed Oct. 7, 1977, now U.S. Pat. No. 4,128,761, by the present inventor is directed to a system for dealing with a typical problem encountered in the fabrication and operation of photodetector assemblies for ballistics use arising because the illumination received by the receiving element is not constant because of varying voltage to the incandescent lamp or lamps, varying amounts of dust in the light transmission path between the light source and the receiving element, aging of the incandescent lamp, varying distances from the light source to the receiving element, and varying cloud and atmospheric conditions and sun angles in the case of sky screens. Reference is made to application Ser. No. 840,254 for a discussion of the electronic aspects of the system which will not be discussed herein, and that application is hereby incorporated by reference.
When using sky screens, the photosensitive elements (normally two in number) view a segment of unobstructed sky through an optical slit. As the projectile passes into the field of view of each photosensitive element, a portion of the light incident thereon is momentarily obstructed, causing a change in the electrical characteristics of each element in sequence. The changes result in electrical signals which are suitably amplified and used to start and stop a timing circuit.
In prior devices of this general type, two significant problems are encountered. The first problem is that, contrary to what might be expected, the light available from a clear sky is significantly less than that available from a cloudy or hazy sky. The reason for this is that the available light for sky screen operation is the diffused light from atmospheric particles and is not the direct light from the sun. While amplifiers preceding the timing circuits can compensate for the reduced light level, at least in part, the system will work better with more light and if the available apparent light level is increased, the range of operation of the system is also increased.
The second problem is that direct light from the sun sometimes reflects from that portion of the projectile, e.g., a bullet, which generally faces the photodetector. This reflected light can be less than, approximately equal to, or greater than the amount of diffused light blocked by the bullet. If the reflected light is either significantly greater than or significantly less than the amount of diffused light blocked by the bullet, circuits such as that disclosed in previously mentioned application Ser. No. 840,254 will respond because such a circuit is designed to detect any perturbation in light level about the ambient level, regardless of the direction of that perturbation. However, in the case in which the reflected light is approximately equal to the amount of diffused light which reaches the photosensitive element in the absence of a projectile, the photosensitive element sees no significant change in the light level when the bullet passes. In other words, the shadow which would be expected to fall on the photosensitive element is essentially obliterated by that reflected light. Because the cancellation of "light with shadow" occurs prior to the point in the system where light levels are converted to electrical signals, there is no way to recover the lost information by using electrical signal processing techniques.